1. Field of the Invention
The present invention relates to electrically actuated clutch and brake mechanisms, and more particularly to an electromagnetically actuated self-adjusting cone clutch with a ball torque booster.
2. Description of the Prior Art
A conventional friction clutch or brake operates on the principle of transmitting torque from an input shaft or driving member to an output shaft or driven member for frictional engagement between the two members to translate torque. Thus, in the case of a clutch, torque is rotatably transmitted from the input shaft to the output shaft. Alternatively, in the case of a brake, torque from the input shaft is absorbed by an output member.
The primary problem associated with friction clutches or brakes is that the level of torque that can be transmitted from the driving member to the driven member is limited by the friction characteristics of the two clutch surfaces, and the engaging force capable of being achieved between the two clutching members is a function of this limitation. If the engaging force is insufficient to transmit a certain level of torque from the driving member to the driven member, slippage may occur at the mating clutch surfaces in spite of the frictional characteristics of the clutch surfaces.
Typically, electromagnetically operated clutches and brakes are operated by an electromagnetic coil which serves to draw the clutching members into driving engagement by virtue of the magnetic attraction between the ferromagnetic members of the clutch. Heretofore, there have existed at least two recognizable types of electromagnetically actuated clutch constructions. One type, herein called the "cone" variation has interengaging friction surfaces between engageable rotatable clutch elements which are conically shaped. By their inherent geometrical configuration, the conical shaped surfaces require a lower axial force to develop sufficient locking of the working faces for coupled rotation between the driving and driven members. The other type of clutch, herein called the "disc" variation, has interengaging friction surfaces which are generally disposed normal to the axis of the engagable rotatable clutch elements. In the disc type designs where the flux path passes through the friction faces the armature pull is entirely axial. The disc type configuration is particularly advantageous due to the flexibility for providing a large axially directed flux path and thereby provide for a strong clutch engaging force between the driven and driving clutch members.
Several prior art designs combine the desirable characteristics of the aforementioned two types of electromagnetic type clutch constructions mentioned above. One such design provides an armature ring element which is generally L-shaped in radial cross section. The element has one annular pole piece with a frusto-conical face and another annular pole piece with a flat disc like face disposed normal to the clutch axis. This design, however, has several drawbacks. For example, since the armature ring element is one piece, there is no means for compensating for wear of either pole piece. Furthermore, the conical friction surface must be made of a magnetic material. In addition, the outer magnetic pole force is almost entirely in the radial direction instead of in the preferred axial direction. Another prior art design uses the same principle as the aforementioned prior art design except that the conical surface is threadably engaged to the disc like pole piece. This design has the added drawback of forcing the electromagnetic flux path through the threads. Designing the flux path to pass through the threads, results in a loss of the generated clutching force produced by the electromagnetic actuating components.
Another prior art design is shown in Miller, U.S. Pat. No. 3,679,034, owned by the assignee of the present application. This design provides a conical frictional element which is moved into engagement with a mutually engageable conical base on an output member by use of a resilient torque transmission member. The resilient torque transmission member provides a sufficient force to disengage the mutually engagable conical surfaces when the electromagnetic force becomes deenergized. The force of the resilient disengaging torque transmission member is in a direction opposite the direction of the force generated by the electromagnetic flux path. Therefore, out of necessity, it requires a greater electromagnetic force to engage the mutually engagable friction torque transmitting faces. Thus, as wear occurs, the electromagnetic force required to engage the frictional surfaces becomes larger in magnitude since the resilient torque transmitting member has to be deflected a greater distance. Furthermore, as wear occurs on the frictional surfaces, adverse wear occurs on the armature face as a result of the centrifugal force acting on the self-adjusting wear compensating members. The centrifugal force delays the action of the self-adjusting wear compensating members thereby permitting the pole face of the output member to come in contact with the rotating armature for a sufficient period of time to cause adverse wear before permitting wear compensation.
In a further effort to increase engagement forces between the clutch members, some prior art designs have included a ball torque boosting arrangement whereby the engagement pressure of the clutching members is increased to augment the electromagnetic engaging force. One such design is shown in U.S. Pat. No. 4,079,821 to Miller, owned by the assignee of the present application. This design is drawn to a single surface electromagnetic clutch or braking device wherein a plurality of spherical members are located in mutually opposing conical recesses between the armature and the output member. The output member abuts a thrust surface so that the armature and the spherical members, which are disposed in the corresponding pockets, cannot move apart from the output member beyond a predetermined axial position. The spherical members are kept in place between the armature and the output member by a biasing member acting on the armature. The armature is moved axially into engagement with the input means when the electromagnetic coil is energized. As the armature moves in the axial direction towards the input means, the distance between the armature and the output member will increase and the spherical members are perimitted to above along the straight sides of the conical recesses in the armature and the output member. Concurrently, after the armature begins to move axially towards the input means, the rotational velocity of the armature is increased by the dynamic condition of the input means. This relative rotational movement between the armature and the output member causes the spherical members to move along the straight surfaces of the conical recesses in both the armature as well as the output member. However, since the output member is restricted from axial movement by a retaining ring, only the armature will move axially in a direction towards the input means. The axial movement of the armature caused by the camming action between the output member and the armature results in a stronger engaging force between the armature and the input means. The engaging force is further enhanced by the face that the torque generated between the magnet body and the armature produces an equal reactive torque in the output member. Again, since the output member cannot move axially, the output member produces an equal but opposite reactive torque resulting in a further axial force which is transmitted back through the spherical members to the magnet body and becomes additive to the electromagnetic engaging force. The overall effect, then, is that the relative rotational movement between the armature and the output member will generate an axial force which is proportional to the generated torque. This axial force is added to the magnet forces, and thereby, causes a net increase in the output torque of the translating device.
While this design has many advantages over simple electromagnetic single surface clutches, namely increasing the engaging force, this design lacks the advantages of the "cone" type single surface clutch. Furthermore, this design does not utilize a friction disc at the engaging surfaces but rather utilizes metal to metal contact. In addition, this design does not provide for an axial thread wear compensation device nor does this design provide for minimizing noise which is caused by backlash of the torque input or output elements.
One prior art design utilizing the electromagnetic clutch or brake having a self-adjusting wear feature and a conical frictional element is shown in U.S. Pat. No. 3,994,379 owned by the assignee of the present application. In this design, a driving friction ring member is threadably engaged to an armature with relative motion in one direction prevented between the armature and the friction ring member by a self-adjusting thread retarder. The fingers of the retarder act upon a narrow surface on the armature to prevent relative motion in one direction between the armature and the friction ring member upon disengaging of the electromagnetic coil. The retarder member only permits the armature to rotate in one direction relative to the friction ring member, that is, as the friction surface wears away. As wear occurs and the electromagnetic coil is energized, the rotating armature and the friction ring member are moved axially toward the pole face. The rotational velocity of the armature is decreased by the static condition of the pole face. Concurrently, the friction ring member, which continues to rotate at the input shaft speed, is caused to move axially forward along the mutually engageable threads between the friction ring member and the armature toward the output member. Thus, the friction surface of the friction ring member moves axially into engagement with its mating friction surface to transmit torque.
This self-adjusting clutch or brake, however, is complex, difficult to assemble and expensive to manufacture. In addition, a substantial amount of heat is generated during the engagement of the frictional elements which causes the threaded connection between the armature and friction member to bind due to thermal expansion. To insure thread adjustment under the most severe thermal growth conditions, it has been found necessary to increase the clearance or tolerance between the threaded surfaces. However, the increased thread clearance permits the armature to drift rotatably along the threads within the friction member. Thus, the operation of the clutch or brake could become uncontrolled and erratic due to the "walking" of the armature within the thread clearance of the friction ring member. This results in the jamming of the armature against the pole faces or severe impacting of the mating frictional surfaces of the clutch or brake.
In summary, none of the above described prior art designs are designed to provide an increase of torque capacity over a single surface conical surface clutch while maintaining the desirable engaging characteristics of a single surface disc type clutch. Furthermore, none of the above designs provides for an instantaneously responding axial thread wear adjuster that prevents "jamming" or wear of the armature against the pole faces. Finally, none of the above prior art designs provide for overcoming the backlash noise associated in driving torque through spherical balls which occurs with unsteady torque inputs or outputs.